1. Field of the Invention
The present invention relates to a radiation detector used for x-ray spectroscopy, for example, and, more particularly, to a circuit for modifying the shape of the output pulse from a radiation detector, such as a semiconductor detector.
2. Description of the Prior Art
A conventional pulse shaping circuit is shown in FIG. 1, where the output signal from a radiation detector 1 is fed to a pulse height analyzer 2 via an integrator 3 and a shaping circuit 4. Specifically, the output signal from the detector 1 is integrated by the integrator 3 and converted into a step voltage signal a (See FIG. 2) proportional to the energy of the incident radiation. The step voltage signal a is shaped into a pulse b having an amplitude proportional to the step voltage by the shaping circuit 4. Pulse b is then fed to the pulse height analyzer 2. The output signal from the radiation detector 1 is shown in FIG. 2(a). The step voltage a delivered from the integrator 3 is shown in FIG. 2(b). When a well known pseudo-Gaussian filter is used as the shaping circuit 4, the output signal b from this Gaussian filter takes a Gaussian waveform as shown in FIG. 2(c).
As shown in FIG. 2(c), it is assumed that radiation falls on the detector 1 at an instant of time t.sub.o. The signal b reaches its maximum value after a lapse of a period T.sub.m. The signal returns to its initial level when a period T.sub.f elapses. The pulse height analyzer 2 makes an analysis of the pulse height when the maximum level is reached.
The next pulse P.sub.2 produced by the radiation detector 1 cannot be measured before a period T.sub.d elapses, T.sub.d =T.sub.f +T.sub.m. The period T.sub.d is called the dead time. The period T.sub.f is equal to the time taken for the pulse b to attenuate below 1/1024 of the maximum level if the pulse height is quantized by means of 1024 quantization levels. Generally, it is necessary that the dead time T.sub.d be several times as long as the period T.sub.m.
V. Radeka has proposed a pulse shaping circuit which has a gated integrator (IEEE, Trans. Nucl. Sci., NS-19, No. 1, 412 (1972)). This shaping circuit is shown in FIG. 3(a), where a gated integrator 5 is inserted between a shaping circuit 4 and a pulse height analyzer 2. A timing controller 6 produces gating signals g.sub.1 and g.sub.2 to the integrator 5. One example of the gated integrator 5 is shown in FIG. 3(b).
Referring to FIG. 3(b), the gated integrator 5 comprises an input capacitor C.sub.1, an operational amplifier and a feedback capacitor C.sub.2. The gated integrator 5 also has switches S.sub.1 and S.sub.2 which are opened and closed in response to gating signals g.sub.1 and g.sub.2, as shown in FIG. 2(d), supplied from the timing controller 6. Therefore, the gated integrator 5 starts integrating its input signal at the instant t.sub.o when radiation falls on the detector 1. After a lapse of the period T.sub.m the Gaussian waveform shown in FIG. 2(c) completely returns to the initial level, and then the switch S.sub.1 is closed, whereby the integration operation ends. The amplitude of the output of the integrator 5 at this time is measured by the pulse height analyzer 2, and then the switch S.sub.2 is closed to discharge a capacitor C.sub.2. Immediately thereafter the output from the integrator 5 goes back to the initial level and so the waveform assumes a shape as shown in FIG. 2(e). As soon as the output settles down to the initial level, the next pulse signal can be measured.
With this pulse shaping circuit shown in FIG. 3(a), signal b is integrated during the measurement and, therefore, noise introduced into signal a is greatly reduced by the averaging action of the integration. Hence, the measuring accuracy is improved.
V. Radeka has also proposed the application of a rectangular wave as shown in FIG. 2(f), instead of the Gaussian waveform shown in FIG. 2(c), to such a gated integrator incorporated in a pulse shaping circuit. The integrator produces a trapezoidal waveform as shown in FIG. 2(g). As the hypotenuse of this trapezoid approaches a straight line, the averaging action approaches an ideal, thus improving the measuring accuracy further.
It is known that a rectangular wave can be created by the use of delay lines, but the resistive component of the delay lines makes it impossible to obtain an ideal rectangular wave. Also, it is not easy to change the delay time. For these reasons, this method has not yet been put into practical use.
F. S. Goulding et al. have proposed that a triangular waveform as shown in FIG. 2(h) be applied to a gated integrator (F. S. Goulding and D. A. Landis, IEEE Trans. Nucl. Sci., NS-29 No. 3, 1125 (1982)). They also have noticed that shortening the dead time T.sub.d induces the associated amplifier to produce more delta noise, and have proposed an index given by the product of noise and the dead time, i.e., N.sub.d.sup.2.T.sub.d, to evaluate the characteristics of this kind of pulse shaping circuit. This has been considered highly valuable. N.sub.d.sup.2, called a delta noise index, is given by: EQU N.sub.d.sup.2 =.sup.n .intg..sub.o.sup..infin. [F(t)].sup.2 dt
where F (t) is the waveform of the output from a radiation detector. It is considered that as the value of the product N.sub.d.sup.-2.T.sub.d decreases, the characteristics of the pulse shaping circuit are improved.
The theoretically optimum values of the product N.sub.d.sup.-2.T.sub.d of various pulse shaping circuits are listed below in Table 1.
TABLE 1 ______________________________________ Pulse Shaping Circuit (1) (2) (3) (4) ______________________________________ .sup.--N.sub.d.sup.2.T.sub. d 9.4 7.35 6 5 ______________________________________
In Table 1, pulse shaping circuit (1) is the conventional circuit shown in FIG. 1. Pulse shaping circuit (2) is the already proposed circuit which uses a gated integrator as shown in FIG. 3(a). Circuit (3) utilizes a triangular waveform as proposed by F. S. Goulding et al. Circuit (4) employs a rectangular waveform as proposed by V. Radeka. In the cases where a gated integrator is used, T.sub.d is given by T.sub.d =Tm.sub.1 +T.sub.r, where Tm.sub.1 is the time period when S.sub.1 is open and T.sub.r is the recovery time as shown in FIG. 2(d).
As can be seen from this table, the use of a rectangular waveform theoretically gives rise to the best value. However, this method has not been put into practical use as mentioned previously.